<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style lang="css">
        .container {
            display: flex;
            height: 500px;
            background: #ccc;
            font-size: 50pt;
            color: #fff;
        }

        .left {
            flex: 1;
            background-color: red;
        }

        .center {
            flex: 100px;
            background-color: blue;
        }

        .right {
            flex: 1;
            background-color: green;
        }
    </style>
</head>

<body>
    <div class="container">
        <div class="left">left</div>
        <div class="center">center</div>
        <div class="right">right</div>
    </div>
</body>

</html>